Unraveling and leveraging in situ surface amorphization for enhanced hydrogen evolution reaction in alkaline media

Surface amorphization provides electrocatalysts with more active sites and flexibility. However, there is still a lack of experimental observations and mechanistic explanations for the in situ amorphization process and its crucial role. Herein, we propose the concept that by in situ reconstructed amorphous surface, metal phosphorus trichalcogenides could intrinsically offer better catalytic performance for the alkaline hydrogen production. Trace Ru (0.81 wt.%) is doped into NiPS3 nanosheets for alkaline hydrogen production. Using in situ electrochemical transmission electron microscopy technique, we confirmed the amorphization process occurred on the edges of NiPS3 is critical for achieving superior activity. Comprehensive characterizations and theoretical calculations reveal Ru primarily stabilized at edges of NiPS3 through in situ formed amorphous layer containing bridging S22− species, which can effectively reduce the reaction energy barrier. This work emphasizes the critical role of in situ formed active layer and suggests its potential for optimizing catalytic activities of electrocatalysts.

1. Could the surface amorphization of the catalyst be controlled accurately through the method provided in this paper? The detailed experimental process should be provided for others to repeat. 2. Does the samples immerse in RuCl3 solution for different durations affect the loading amount of Ru in the nanosheets. Is the Ru-NiPS3 with 0.81wt% content of Ru the best one after optimization? The related information should be provided. 3. Why are the Ru atoms mainly distributed at the edge of the NSs, forming a Ru-enriched shell? What is the mechanism in forming such kind of structure? 4. The authors demonstrated that the Ru0 or Ru cluster was unstable during the HER test, and only Ru4+ species remained at the edge sites of NiPS3 NSs (Fig. 5j). So, are these Ru0 or Ru cluster species converted to Ru4+ or dissolved in the electrolyte? Further experimental verification is suggested. 5. Is all Ru scattered around the edges after electrolysis? Does the amorphous state form only on the [001] crystal plane ( Fig. 5a)? Or are there amorphous states in other crystal planes? It is recommended to supplement other regional HAADF images. 6. From Fig. 4 it is hesitant to conclude anything from the in situ electrochemical liquid cell TEM holder. The TEM images and the contrast are not convincing enough. 7. In the XPS, the same element fitting should be redone with all components having the same FWHM. In Figure 5 should be refitted it.
Reviewer #4 (Remarks to the Author): In this work, the authors aim to address a commonly overlooked issue in the alkaline HER: the formation of an amorphous layer during the reaction process. The lack of evidence for the in-situ characterization of the amorphous layer formed during the reaction has hindered the full development of its essential role and potential applications. Fu et al. have utilized an advanced in-situ liquid TEM technique to directly demonstrate the formation of the amorphous layer, which provided direct evidence of the amorphization. Their results prove that the amorphous layer is significant in analyzing the true catalytic mechanism and active sites for the HER process. Furthermore, the authors have also shown, both theoretically and experimentally, that by rationally designing and utilizing the unavoidable amorphous layer, the catalytic performance of the electrocatalyst can be significantly improved. I highly recommend this manuscript for publication in Nature Communications. Please find below some detailed comments for the authors to consider. 1. The manuscript was written mainly to discuss the amorphous layer, will the sample be oxidized during the storage? And if the sample was oxidized after the preparation, will the oxidized layer have any influence on the catalytic process? 2. As shown in Figure 2g, some Ru(0) was detected before the HER test. I think this should belong to the Ru cluster or Ru metal. Will the Ru(0) have any influence on the catalytic performance? 3. The in-situ TEM images and movies have demonstrated that the NiPS3 NSs undergo an amorphization process. Is it possible that the electrolyte could have an impact on the formation of the amorphous layer on the catalyst surface? 4. In Figure 1e, the author used a CIF profile to demonstrate the crystal structure of NiPS3. It would be better if changed it into a HAADF-STEM simulation image. 5. Some typos in the manuscript should be corrected. For instance, in line 57, the word "lead" should be "leads". In line 115" characterization is conducted" should be "characterization was conducted". In line 355, it is recommended to spell out the abbreviation "DI" in its complete form, when it is used for the first time in a document.

Reviewer #1 (Remarks to the Author):
In the submitted paper, Fu et al. presented results regarding the synthesis of Ru-NiPS3 nanosheets, their characterization and electrochemical properties toward HER. They also proposed that the property improvement observed can be attributed to amorphization of the nanosheets. Here, I will mainly comment on the electron microscopy data (both ex situ and in situ) and their interpretations, which are both highly problematic.
1. The substitution of Ru into the lattice cannot be directed elucidated from the intensity profiles as the authors did in Figure 1f and Supplementary Figure 11. The samples are too thick for such a simple interpretation. Here, there are two common ways to infer the presence of heavier atoms.
a. Look at the samples that are not under strong channeling conditions (i.e. absence of any atomic resolution in the support). However, it also means that there is no way to determine if the Ru simply sits on the surface or substitutes for Ni b. The other way is to perform image simulations for comparison to show that the intensity difference is indeed consistent with the substitution of a Ni atom by a Ru atom.
2. The liquid cell experiments are interpreted wrongly and have significant issues. a. The electron flux used for imaging is a critical experimental that needs to be declared in every paper using the technique, which is currently absent in the paper.
b. The experiments also appeared to be performed under thin liquid conditions, which is not ideal for electrochemistry experiments. The authors should at least mention that they did not work with a fully filled cell.
c. The amorphization cannot be inferred from the electron diffraction patterns. It is clear from Supplementary Movie 1, the NS is rotating/rolling during the experiment (also from the fading in and out of diffraction contrast). The loss of intensity is due to the NS moving out of strong diffraction conditions and not because the sample became amorphous.
R-2 d. The center diffraction spot in 4(d) is also highly stigmatic (i.e. not circular), the diffraction rings acquired under such conditions will be elliptical and not suitable for reliable indexing.
e. It is also common practice to use a beam blocker to blank the central beam. The main reason (apart from avoiding damage to the camera) is that with an intense central beam takes up most of the dynamic range of the detector, which makes it very difficult to pick up weaker diffraction spots.
3. Hence, the entire discussion on page 10 regarding the difference between the behavior on Pt and on GC is (likely) wrong. The more plausible hypothesis is that the formation of hydrogen bubbles on the Pt pushes the NS and causes it to move or curl whereas the sample on the GC is stagnant.
4. Did the authors acquire DPs from the sample on GC?
5. The author's claim of amorphization based on liquid cell data is, in fact, not selfconsistent with their own ex situ analysis.
a. The AC-STEM shows that most of the NS remains crystalline and any amorphization is superficial.
b. This is also supported by the negligible difference powder diffraction patterns.
Other comments 6. The authors should mention in the main text the average thickness of the NSs.
7. Line 110. The authors specify that that the d-spacings for are the (130) and (1-30) of NiPS3.
8. Why are the chronopotentiometry experiments (@100 mA and @5 nA) fixed at positive current densities? Shouldn't the currents? 9. Authors should also provide the chronopotentiometry data from the liquid cell holder.

Reviewer #2 (Remarks to the Author):
In this manuscript, the authors synthesized a Ru-NiPS3 nanosheets  catalyst by three-step procedure for the hydrogen evolution reaction (HER). The samples were characterized by SEM, EDS, XPS, XRD and in situ TEM. The catalyst properties have been systematically discussed through electrochemical test and DFT, which well-demonstrates the importance of the highly active amorphous surfaces. Thus, publication this work on nature communications could be recommended after carefully addressing the following issues regarding material characterizations and proposed mechanism.
1. Could the surface amorphization of the catalyst be controlled accurately through the method provided in this paper? The detailed experimental process should be provided for others to repeat.
2. Does the samples immerse in RuCl3 solution for different durations affect the loading amount of Ru in the nanosheets. Is the Ru-NiPS3 with 0.81wt% content of Ru the best one after optimization? The related information should be provided.
3. Why are the Ru atoms mainly distributed at the edge of the NSs, forming a Ruenriched shell? What is the mechanism in forming such kind of structure? 4. The authors demonstrated that the Ru0 or Ru cluster was unstable during the HER test, and only Ru4+ species remained at the edge sites of NiPS3 NSs (Fig. 5j). So, are these Ru0 or Ru cluster species converted to Ru4+ or dissolved in the electrolyte?
Further experimental verification is suggested. 5. Is all Ru scattered around the edges after electrolysis? Does the amorphous state form only on the [001] crystal plane (Fig. 5a)? Or are there amorphous states in other crystal planes? It is recommended to supplement other regional HAADF images.
6. From Fig. 4 it is hesitant to conclude anything from the in situ electrochemical liquid cell TEM holder. The TEM images and the contrast are not convincing enough. 7. In the XPS, the same element fitting should be redone with all components having the same FWHM. In Figure 5 should be refitted it.

Reviewer #4 (Remarks to the Author):
In this work, the authors aim to address a commonly overlooked issue in the alkaline HER: the formation of an amorphous layer during the reaction process. The lack of evidence for the in-situ characterization of the amorphous layer formed during the reaction has hindered the full development of its essential role and potential applications. Fu et al. have utilized an advanced in-situ liquid TEM technique to directly demonstrate the formation of the amorphous layer, which provided direct evidence of the amorphization. Their results prove that the amorphous layer is significant in analyzing the true catalytic mechanism and active sites for the HER process.
Furthermore, the authors have also shown, both theoretically and experimentally, that by rationally designing and utilizing the unavoidable amorphous layer, the catalytic performance of the electrocatalyst can be significantly improved. I highly recommend this manuscript for publication in Nature Communications. Please find below some detailed comments for the authors to consider.
1. The manuscript was written mainly to discuss the amorphous layer, will the sample be oxidized during the storage? And if the sample was oxidized after the preparation, will the oxidized layer have any influence on the catalytic process? 2. As shown in Figure 2g, some Ru(0) was detected before the HER test. I think this should belong to the Ru cluster or Ru metal. Will the Ru(0) have any influence on the catalytic performance?
3. The in-situ TEM images and movies have demonstrated that the NiPS3 NSs undergo an amorphization process. Is it possible that the electrolyte could have an impact on the formation of the amorphous layer on the catalyst surface? 4. In Figure 1e, the author used a CIF profile to demonstrate the crystal structure of NiPS3. It would be better if changed it into a HAADF-STEM simulation image. 5. Some typos in the manuscript should be corrected. For instance, in line 57, the word "lead" should be "leads". In line 115" characterization is conducted" should be "characterization was conducted". In line 355, it is recommended to spell out the abbreviation "DI" in its complete form, when it is used for the first time in a document. We sincerely thank all reviewers for their valuable comments and suggestions, which are certainly helpful for improving the quality of our manuscript. We have thoroughly reviewed and addressed all the points raised by the reviewers in a systematic and careful manner. A point-by-point response to all comments is given below. For clarity, the reviewer's comments are in black text, and our responses are in blue text.
Additions and revisions to the manuscript and SI have been included in this response, and they are given in red text.
Reviewer #1 (Remarks to the Author): In the submitted paper, Fu et al. presented results regarding the synthesis of Ru-NiPS3 nanosheets, their characterization and electrochemical properties toward HER. They also proposed that the property improvement observed can be attributed to amorphization of the nanosheets. Here, I will mainly comment on the electron microscopy data (both ex situ and in situ) and their interpretations, which are both highly problematic.

Response:
We are grateful to the reviewer for the thorough review of our manuscript, and we have carefully considered the feedback regarding the in-situ TEM section. As per the suggestions, we have conducted additional experiments and made significant revisions to this section, which are outlined below.
1. The substitution of Ru into the lattice cannot be directed elucidated from the intensity profiles as the authors did in Fig. 1f and Supplementary Fig. 11. The samples are too thick for such a simple interpretation. Here, there are two common ways to infer the presence of heavier atoms. R-6 a. Look at the samples that are not under strong channeling conditions (i.e. absence of any atomic resolution in the support). However, it also means that there is no way to determine if the Ru simply sits on the surface or substitutes for Ni b. The other way is to perform image simulations for comparison to show that the intensity difference is indeed consistent with the substitution of a Ni atom by a Ru atom.
Response: Thank you for your suggestion. We have incorporated your feedback into the revised manuscript by rearranging the TEM image in Fig   In the manuscript, we added the description of the simulation: Line 118 "……which is consistent with the simulated HAADF image and the corresponding intensity profile R-7 (Fig. 1g, and h) " The simulation detail with Dr. Probe software package is also added: Line 389 "Dr.
Probe was used for simulating STEM-HAADF images. Accelerating voltage, convergence semi-angle, and collection angle were set same as the imaging, which were 300 kV, 15 mrad, and 35-200 mrad, respectively." 2. The liquid cell experiments are interpreted wrongly and have significant issues. a. The electron flux used for imaging is a critical experimental that needs to be declared in every paper using the technique, which is currently absent in the paper.
b. The experiments also appeared to be performed under thin liquid conditions, which is not ideal for electrochemistry experiments. The authors should at least mention that they did not work with a fully filled cell.

Response for a and b:
We sincerely thank the reviewer for bringing to our attention the omission of our experiments. We agree with your perspective that the thin liquid conditions used in our study may differ from the conditions of a realistic electrochemical cell. However, the use of a "thin liquid strategy" is a commonly applied technique to obtain high-quality TEM images in liquid cells, as demonstrated in previous works (e.g., ACS Nano 2021, 15, 6, 10228-10240, J. Am. Chem. Soc. 2022, 144, 34, 15698-15708, Nature 614, 262-269 (2023). Therefore, we followed this established approach to perform our in-situ liquid TEM measurements.
In the revised manuscript, we have emphasized this point in the Method section and provided more experimental information. We also provided additional experimental results of the in-situ TEM measurements. The experimental section has been revised and is presented below: What should be mentioned is that, we adopted a currently commonly used "thin liquid strategy" to minimize the influence of liquid on the resolution of TEM, so that we can acquire some sufficiently clear TEM images during our experiments.

In situ liquid electrochemical TEM characterization
c. The amorphization cannot be inferred from the electron diffraction patterns. It is clear from Supplementary Movie 1, the NS is rotating/rolling during the experiment (also from the fading in and out of diffraction contrast). The loss of intensity is due to the NS moving out of strong diffraction conditions and not because the sample became amorphous.
d. The center diffraction spot in 4(d) is also highly stigmatic (i.e. not circular), the diffraction rings acquired under such conditions will be elliptical and not suitable for reliable indexing.
e. It is also common practice to use a beam blocker to blank the central beam. The main reason (apart from avoiding damage to the camera) is that with an intense central beam takes up most of the dynamic range of the detector, which makes it very difficult to pick up weaker diffraction spots.

Response for c, d and e: Thank you for your insightful comment. Regarding the in situ
liquid TEM cell, we would like to acknowledge that the distribution of the sample on the electrodes is randomized, and the selection of the investigated sample is sometimes restricted. In our previous attempt at in situ liquid TEM measurement, we chose a relatively thin sample on the Pt electrode, as we speculated that the thinner sample would react more easily and provide a more intuitive understanding of the amorphization process. However, we agree with the reviewer's suggestion that there may be two shortcomings with this approach. Firstly, the Pt electrode is highly active R-9 for HER, and the generated H2 bubble around the Pt electrode may render the nanosheets unstable and provide inaccurate information. Secondly, the movement of the nanosheet makes it difficult to obtain an ideal SAED pattern.
Therefore, in the revised version, we conducted new round of in-situ liquid TEM measurement and have optimized the experimental conditions: To minimize the potential impact of sample movement on our TEM imaging, we opted to use a relatively thick sample. Additionally, we carefully selected a sample located at the GC electrode, which we believe will allow us to obtain higher-quality TEM images. The final results are demonstrated in Fig. R1-2, and we replace the Fig.   4 in the revised manuscript with the modified one. 3. Hence, the entire discussion on page 10 regarding the difference between the behavior on Pt and on GC is (likely) wrong. The more plausible hypothesis is that the formation of hydrogen bubbles on the Pt pushes the NS and causes it to move or curl whereas the sample on the GC is stagnant.

Response:
We thank the reviewer for pointing out this issue. We have revised the discussion on the in situ electrochemical liquid cell TEM results based on the feedback from the reviewer. The modified discussion takes into account our new in situ liquid TEM measurement results, which are presented below, and the corresponding Fig. 4 has already presented as Fig Fig. 24). The in situ TEM images before and after 2 h continually chronopotentiometry test showed significant changes and the corresponding SAED patterns also exhibit polycrystalline and amorphous rings after HER test in alkaline electrolyte ( Fig. 4b and 4c). Detailed b. This is also supported by the negligible difference powder diffraction patterns.

Other comments
Response: We thank the reviewer for the comments. For our Ru-NiPS3 sample, there are two main reasons that limit the observation of the amorphization process, that is the thickness of the nanosheet and the reaction time. In previous version, we tried to obtain a faster and obvious variation of the structure of the nanosheets, and to satisfy the requirement, we chose a relatively thin sample located on the Pt electrode (like demonstrated in the previous manuscript). We agree the reviewer that it may bring two difficulties to our observations, one is the thinner nanosheet would be amorphized rapidly, another is the influence of H2 bubbles may make the result implausible. After R-13 careful consideration, in our revised version, the new in situ liquid TEM measurements were further modified and optimized.
Firstly, we selected a representative sample with a relatively thick thickness, which is close to the average thickness of the synthesized nanosheets of this work. Thus, the evolution of the structure is slower, which is conducive to long-term observation.
Secondly, the sample we chose was located on the GC electrode, which is far away from the Pt electrode and minimize the impact of liquid disturbance caused by the bubbles during the reaction process.
After several trials, we obtained the following sequential TEM images (Fig. R1-4) and the corresponding SAED patterns (Fig. R1-5). As demonstrated in the Fig. R1-4 and R1-5, the overall crystallinity is still good, and the edge positions have more amorphization, which is proved by the gradual formation of amorphous ring in SAED.
We further collected SAED patterns of more edge positions, which all consistently indicated that, at the thinner regions, amorphization has already taken place in the initial stages of the reaction (Fig. R1-6).  6. The authors should mention in the main text the average thickness of the NSs.

Response:
We would like to express our gratitude to the reviewer for their important comments. We would like to clarify that we did calculate the average thickness of our sample based on the SEM image in Supplementary Fig. 4, which was found to be approximately 150 nm. We have included this information in the revised manuscript (line 99). To further confirm the thickness results with higher accuracy, we also used atomic force microscopy (AFM) to analyze the thickness of our sample in revised manuscript.  Response: We thanks the reviewer for the comment. We indeed made mistakes on the crystal plane, which according to the HRTEM image and the corresponding FFT image ( Fig. R1-8) should be (130) and (1 ̅ 30), respectively. We apologize again for this mistake and have made the necessary corrections in the revised manuscript (Line 114). Response: Thanks for comment. This two chronopotentiometry measurements are used to express different concepts. In the macroscopic three-electrode electrolyzer system, the directly measured quantity is indeed the current. But this is not a suitable parameter for comparing the catalytic performance in different electrocatalysts. For example, a current of 100 mA is much larger than 10 mA, but the former one may be obtained on a larger electrode. Thus, current density, which is calculated by dividing the current by the geometric area of the working electrode, is more reliable for comparison. So, in the realistic three-electrode electrolyzer system, we usually use current density (mA cm -2 ) to demonstrate the electrocatalytic performance. However, in the in-situ liquid TEM cell, it is very hard to count the actual area or loading of catalyst falling on the electrodes.

R-16
Considering that in situ liquid-phase TEM mainly studies the catalytic reaction mechanism and directly observes the reaction process, a simple current application is sufficient to initiate the reaction. That is why we use current density and current to describe the reaction condition in macroscopic three-electrode electrolyzer system and liquid-phase TEM measurements, respectively.
We also would like to clarify that we did not fix the parameter at a positive current density of 100 mA cm -2 and a current of 5 nA. It is evident that for HER, the current R-17 measured during the reaction should be negative. The current densities and currents expressed in the paper are taken as absolute values, which is a common way to demonstrate HER current density (e.g., Nat. Commun. 10, 3899 (2019), Energy Environ. Sci., 2019,12, 3522-3529, Nano-Micro Lett. 15, 120 (2023). To eliminate any ambiguity, we have changed the current density to -100 mA cm -2 in Fig. 3g in the revised version. The modified figure is presented below (Fig. R1-9). (g) Chronoamperometry curve test for Ru-NiPS3 NSs at a fixed current density of -100 mA cm -2 9. Authors should also provide the chronopotentiometry data from the liquid cell holder.
Response: Thanks for the suggestion. In the revised version, we added the chronopotentiometry data as Supplementary Fig. 24, which replaced the initial Fig. in the original version to demonstrate more clear structure of the in situ liquid TEM chip.

R-19
Reviewer #2 (Remarks to the Author): In this manuscript, the authors synthesized a Ru-NiPS3 nanosheets  catalyst by three-step procedure for the hydrogen evolution reaction (HER). The samples were characterized by SEM, EDS, XPS, XRD and in situ TEM. The catalyst properties have been systematically discussed through electrochemical test and DFT, which well-demonstrates the importance of the highly active amorphous surfaces. Thus, publication this work on nature communications could be recommended after carefully addressing the following issues regarding material characterizations and proposed mechanism.
Response: We appreciate the reviewer's favorable feedback on our manuscript and have made extensive revisions based on suggestions.
1. Could the surface amorphization of the catalyst be controlled accurately through the method provided in this paper? The detailed experimental process should be provided for others to repeat.
Response: Thanks for the suggestion. In fact, the surface amorphized layer was formed gradually during the HER process. The process is hard to be accurately controlled at this stage. There are two reasons: 1) the nanosheets are grown on carbon cloth desultorily with hydrothermal method, thus, the contact with electrolyte may not be exactly the same. 2) as demonstrated in the SEM image, the large contact areas between all the nanosheets may hinder sufficient contact between the electrolyte and the contacted regions, thereby delaying the amorphization process.
However, the thickness of the amorphous layer would gradually stabilize. It has been proved that, the amorphous layer can also protect the inner part of the material, which ensures the stability of the structure (ACS Catal., 2018, 8(5): 4091-4102, Matter, 2021, 4(9): 2850-2873. Thus, when the thickness of the amorphous layer grown to a certain level, the inner part would be separated from the electrolyte, which in return stabilize the materials for long term use. As shown in Fig. R2-1, we further obtain the HRTEM images which demonstrated that the thickness of the amorphous layer in the Ru-NiPS3 nanosheet remained stable during the long-term stability testing process. It should be R-20 mentioned that the amorphous layer is generated gradually during the HER stability test. After the initial activation step is completed, the catalytic performance of the material is roughly determined. The increased thickness mainly results in better protection of the inner part, which ensures the structural stability of the nanosheet. We have also included more details about the preparation protocol for the electrode in the Methods section. The revised section is presented below (the part highlighted in yellow is the modified content):

Electrochemical measurements
All electrochemical measurements were conducted using a typical three-electrode cell with a CHI 760E electrochemical workstation (CH Instruments, Inc. Shanghai 2. Does the samples immerse in RuCl3 solution for different durations affect the loading amount of Ru in the nanosheets. Is the Ru-NiPS3 with 0.81wt% content of Ru the best one after optimization? The related information should be provided.

Response:
We thank the reviewer for the useful suggestion. Compared with other doping method, ion-exchange is a relatively slow process. However, this method is a simple and scalable technique, which could effectively reduce the waste of precious metals, and realize single atom doping.
Under the conditions of our laboratory, we selected 5 different dipping interval times to demonstrate our proposal (that is 0.5 h, 2 h, 4 h, 16 h, and 20 h). The HER activities of these five electrodes are shown in Supplementary Fig. 16., and we also put it here as Fig. R2-2 ). As demonstrated, when the dipping time reached 16 h~20 h, the activities of the electrode are almost the same. Considering the time issue, we then choose 16 h as the representative sample fur further study, and the Ru doping amount of this sample is about 0.81%. We assume that after dipping 16 h~20 h, the ion-exchange process will reach equilibrium, and the catalytic performance remain unchanged like shown in Fig.

R2-2.
With the growth of the immersion time, the activities of the electrode may be better due to more Ru was loaded into the NiPS3 lattice. However, the current experiments can already prove the concept we are focusing on.  Table R2-1. According to the ICP-OES results, a concentration of 0.8% is considered as the maximum limit for ion exchange doping of Ru atom into the NiP3 NSs 3. Why are the Ru atoms mainly distributed at the edge of the NSs, forming a Ruenriched shell? What is the mechanism in forming such kind of structure?

R-22
Response: Thank you for your insightful comment. We will address the question from two different perspectives.
First, the formation of amorphous layer in alkaline media is inevitable during the HER process for most transition metal based electrocatalyst (e.g., Angew. Chem. Int. Ed. 2018, Adv. Mater., 2021. However, the formed amorphous layer could provide effective protection for interior electrodes (Matter, 2021, 4(9): 2850-2873), and with the reaction progressing, the amorphous layer would protect the inner part from contacting of the electrolyte, which would stabilize the core-shell structure.
Thus, the reaction primarily occurs at the outer amorphous layer, making it a key factor in determining the catalytic activity and efficiency of the electrocatalyst, and the modification (such as increase the specific surface area, increase the number of active sites, and so on) is mainly focused on the amorphous layer.
Secondly, the amorphous layer contains many bridging S2 2species, and many dangling bonds, which contribute to the adsorption of Ru species. Our DFT calculation results also indicated that Ru bonded with S2 2species is more stable than other possible position ( Fig. R2-3, also demonstrated in Supplementary Table 5), and tend to form stable Ru rich edges for reaction. 4. The authors demonstrated that the Ru0 or Ru cluster was unstable during the HER test, and only Ru4+ species remained at the edge sites of NiPS3 NSs (Fig. 5j). So, are these Ru0 or Ru cluster species converted to Ru4+ or dissolved in the electrolyte?
Further experimental verification is suggested.

Response:
We thank the reviewer for the comment. To verify this, we further measured the ICP-OES of the electrolyte after different reaction duration. As shown in Fig. R2-4, the content of Ru species remained almost unchanged after test for 2 h, which proved the unstable Ru 0 species would dissolve in the electrolyte. The Fig. R2-4 is also included in the Supplementary Information as Supplementary Fig.30. The manuscript is also modified: Line 293 "Further ICP-OES measurements confirmed that the Ru species gradually dissolved into the electrolyte, and the remaining Ru 4+ species served as the active species for the HER process ( Supplementary Fig. 30 planes? It is recommended to supplement other regional HAADF images.

Response:
We appreciate the reviewer's comment. The Ru atoms were randomly doped into the NiPS3 lattice, which means that they may exist both at the edges and in the inner part of the nanosheets. However, during the self-reconstruction process, Ru atoms may gradually leach out like other metal cations. The amorphous layer, therefore, would serve as an absorbent for Ru atoms to reduce the leaching of Ru. Although Ru atoms may exist in the inner crystalline part, the HER process would only occur at the surface or edges of the nanosheets. This means that only the Ru atoms at the amorphous layer can contact with the electrolyte and serve as the active sites for HER.
In fact, the amorphization process did not depend on the specific crystal plane, and the edges of the nanosheets may undergo a certain degree of amorphization process.
We further acquired the AC-HAADF STEM image from other regions to demonstrate this. The details are shown in Fig. R2-5 (also added into the Supplementary Information as Supplementary Fig. 29). showed that the amorphization process is independent of the crystal plane orientation.
6. From Fig. 4 it is hesitant to conclude anything from the in situ electrochemical liquid R-26 cell TEM holder. The TEM images and the contrast are not convincing enough.

Response:
We are grateful to the reviewer for drawing our attention to the shortcomings in our previous in-situ TEM experiments. After careful consideration and analysis of the possible issues, we conducted new, additional in-situ TEM experiments, and the results are presented as Fig. R2-6, and we also replaced the initial version of Fig. 4 with the revised figure. As demonstrated in Supplementary Fig. 23a, the morphology demonstrated negligible variation after the immersing process, which is also proved by the corresponding XRD pattern (Supplementary Fig. 23b) and Raman spectra (Supplementary Fig. 23c) Fig. 24). The in situ TEM images before and after 2 h continually chronopotentiometry test showed significant changes and the corresponding SAED patterns also exhibit polycrystalline and amorphous rings after HER test in alkaline electrolyte (Fig. 4b and 4c) Fig. 25 and 26). It is also founded that the amorphization process would be more obvious at the thinner edges, which contribute to the formation of the functional amorphous layer for electrochemical reaction (Supplementary Fig. 27

28, and Supplementary Movie S3)."
From in situ liquid-phase transmission electron microscopy, we can draw the following conclusions: (1) The in-situ liquid TEM sample are demonstrated as Fig. R2-7a. And the morphology variation of the edge site (orange square) is demonstrated in Fig. R2-7b, which shows significant amorphization process, especially at the edges. Relative thin nanosheets will be amorphized faster (red square, Fig. R2-7c). Thicker regions, on the other hand, the crystallinity is better maintained, which further proved the thinner edges at the nanosheets would be easily amorphized.
R-29 2) The main part of the sample remained relatively stable throughout the reaction process according to the SAED pattern ( Fig. R2-8). However, we then reaction reached about 75 min, we can also observe the formation of polycrystal rings and amorphous rings generated with the reaction going on. This result further confirmed the amorphous process will mainly happen on the edges or surfaces without destroying the structure of nanosheets, which is also proved by the XRD and XPS results obtained after stability test (Fig. 5).

Response:
We thank the reviewer for the comment. According to the reviewer's suggestion, we refitted the XPS data in Fig. 5. In the revised figure, we keep the FWHM R-30 of the fitting components close to each other in the same element. Details are as follows ( Fig. R2-9). We also changed the description of in the manuscript about the XPS result of the S 2p spectra, since the broad peak located around 170 eV was deconvoluted into two different peaks which belongs to the sulfate species [SO4] 2-(at ~170.8 eV) and sulfite [SO3] 2-(at ~168.5 eV) The description of Fig.5i is also modified: From Line 351 "Another broad peak at ~ 169.6 eV was deconvoluted into two different peaks which belongs to the sulfate species [SO4] 2-(at ~170.8 eV) and sulfite species [SO3] 2-(at ~168.5 eV) (Fig. 5i)." And the revised Fig. 5 in the main text are as follows (Fig. R2-10): R-31 NSs after HER test.

R-32
Reviewer #4 (Remarks to the Author): In this work, the authors aim to address a commonly overlooked issue in the alkaline HER: the formation of an amorphous layer during the reaction process. The lack of evidence for the in-situ characterization of the amorphous layer formed during the reaction has hindered the full development of its essential role and potential applications. Fu et al. have utilized an advanced in-situ liquid TEM technique to directly demonstrate the formation of the amorphous layer, which provided direct evidence of the amorphization. Their results prove that the amorphous layer is significant in analyzing the true catalytic mechanism and active sites for the HER process.
Furthermore, the authors have also shown, both theoretically and experimentally, that by rationally designing and utilizing the unavoidable amorphous layer, the catalytic performance of the electrocatalyst can be significantly improved. I highly recommend this manuscript for publication in Nature Communications. Please find below some detailed comments for the authors to consider.

Response:
We thank reviewer for his positive comments on our manuscript.
1. The manuscript was written mainly to discuss the amorphous layer, will the sample be oxidized during the storage? And if the sample was oxidized after the preparation, will the oxidized layer have any influence on the catalytic process?
Response: Thanks for the comment. Indeed, the oxidation of transitional metal based electrocatalysts is sometimes inevitable in the atmosphere, which has been already recognized by some previous works, such as Angew. Chem. Int. Ed., 54: 14710-14714., Nat Commun 7, 13216 (2016), Nat. Energy 6, 1144-1153(2021), Nat Commun. 12, 3540 (2021, and so on. This phenomenon also suits the situation in our case, which could be confirmed by the HRTEM images in Fig. 1b and Supplementary Fig. 7. It can also be seen that the thickness edges of the NiPS3 and Ru-NiPS3 NSs are similar (less than 1 nm). Therefore, if any influence of inevitable oxidation occurred due to exposure to air, it should be similar for both samples.
Despite this possibility, we observed an obvious increase in HER activity. Based on this evidence, we can conclude that the enhancement is due to the proposed Ru-enriched  5. Some typos in the manuscript should be corrected. For instance, in line 57, the word "lead" should be "leads". In line 115" characterization is conducted" should be "characterization was conducted". In line 355, it is recommended to spell out the abbreviation "DI" in its complete form, when it is used for the first time in a document.
Response: Thanks for the comments. We have carefully reviewed our work and made further revisions to address the typos and grammar mistakes, including those that the reviewer pointed out. 2. It will be inaccurate to describe the sample after 125 min as a polycrystal as the term implies a randomly oriented structure. The diffraction pattern in 4e clearly shows that the structure is not random and that clearly orientational relationships within the larger nanosheet flake. I suggest that the authors revisit this section and update the discussion accordingly.
Response: Thanks for your comments. We agree with the reviewer that even after 125 min test, the nanosheet still mainly keep the orientational relationships within the larger nanosheet flake. In our previous description, there were inaccuracies that led to misunderstandings. We would like to clarify our original intention, which was to convey that during the reaction process, there is a localized amorphization and polycrystallization at the edges of the nanosheets. However, it is important to note that the overall crystallinity of the nanosheets remains largely unaffected, as evidenced by the distinct crystalline nature observed in the SAED patterns. We apologize for any confusion caused by our previous wording. To eliminate any ambiguity in the description, we have made the following modifications: (Page10, Line 252-258) "After continuously subjecting the Ru-NiPS3 NSs to a 2-hour chronopotentiometry test, a significant reconstruction was observed at the edge position of the NSs (Figure 4b and 4c). The corresponding selected area electron diffraction (SAED) patterns clearly showed that while most of the NSs remained unchanged after the chronopotentiometry test(with similar diffraction spots as in Figure 4d), a portion of the nanosheet underwent a transformation into a polycrystalline or amorphous state during the reconstruction process (as indicated by the presence of faint polycrystalline rings and amorphous halo ring in Figure 4e) Figure 4c (as demonstrated in Figure R1-1).
As for the comparison with the ex situ results after similar times, it may indeed be inappropriate to directly compare in situ liquid-phase TEM images with non-in situ high-resolution TEM (HRTEM) images. This is because these two techniques differ in sample preparation and environmental conditions.
In situ liquid-phase TEM is typically used to study the structure and dynamic behavior of materials in liquid environments. It provides information about materials under in situ conditions, but image quality may be limited due to factors such as resolution and contrast, which can be influenced by the presence of the liquid environment.
On the other hand, ex situ HRTEM is commonly used to investigate the crystal structure and details of materials. It is conducted under vacuum or dry conditions and can achieve higher resolution and clearer images.
As shown in Figure 4 and Supplementary Fig. 26 and Fig.27, The in situ TEM images obtained in liquid phase using our in situ facility can only achieve resolutions on the scale of few tens of nanometers, which is insufficient for clearly studying the thickness of amorphous layers (which, as revealed by ex situ HRTEM image, is less than 10 nm).
Considering the significant differences between the testing conditions in in situ liquidphase TEM and the actual electrolytic cell reactions, we are inclined to utilize in situ liquid-phase TEM for investigating the overall structural evolution of the nanosheets, Response: Thank you for the reviewer's suggestions, and we sincerely apologize for the confusion caused. In Figure 28, we present the changes in the thickness of the edge amorphous layer of Ru-NiPS3 nanosheets with increasing HER reaction time. The label 'dipping for 16h' following Ru-NiPS3 in the caption refers to the sample that underwent ion exchange for 16 hours (details are demonstrated in Supplementary Fig.13-Fig. 14,   Fig. 16-Fig. 17), representing the typical sample as described in our paper. It is important to note that the ' dipping for 16h ' label in the caption does not correspond to the duration of the electrochemical reaction depicted in the figure. We apologize for any confusion caused by this discrepancy.
To eliminate any ambiguity, we have made the following modifications to the main text  5. Response to my previous comment 2(b): "What should be mentioned is that, we adopted a currently commonly used "thin liquid strategy" to minimize the influence of liquid on the resolution of TEM, so that we can acquire some sufficiently clear TEM images during our experiments." Just because an approach has been used in prior work does not mean that does not have its issues. For example, early work in the field of liquid cell TEM commonly used electron dose rates of hundreds to even thousands of electrons/A2/s but now, we know that such dose rates are not tenable for realistic experiments. Here, I suggest that the authors rephrase the sentence in the methods (page 16, line 406-408) to something similar to what I have outlined below.
"For these in situ liquid TEM experiments, the samples were imaged within a thin liquid layer so that we can acquire sufficiently clear TEM images with good spatial resolution.
In this case, one should note that the exact applied conditions are not identical to that of a realistic electrochemical cell, which may lead to differences between the results obtained from in situ and ex situ measurements.
Response: Thank you for providing us with such detailed and constructive feedback.
We completely agree with your point that even commonly used techniques can have certain issues. Therefore, we cannot use this as a justification for describing the selection of experimental conditions. Under the circumstances of our paper, a more appropriate approach to describing the experimental conditions would be to provide an objective description rather than basing it solely on the fact that the method has been used before. Based on the template you provided, we have made the following modifications to the relevant section: ( We would like to express our heartfelt gratitude once again for the meticulous and detailed references provided by the reviewer.

Reviewer #2 (Remarks to the Author):
In this manuscript, the authors applied trace Ru doped NiPS3 nanosheets for highefficiency alkaline hydrogen evolution reaction. Catalyst properties have been systematically discussed through electrochemical test and compared experiments without obvious errors. Stability tests are specific and persuasive. In particular, the feedback on the in situ TEM section is very detailed and easier to understand. I think the quality of this manuscript has been enhanced considerably after the revision. After addressing some points, this his manuscript can be considered for publication in Nat. Commun.

Response:
We greatly appreciate the recognition and constructive feedback provided by the reviewer on our work.
1. The XPS survey spectrum for Ru-NiPS3 NSs should be provided.
Response: Thanks for your suggestion. We have added the XPS survey spectrum of Ru-NiPS3 NSs as demonstrated in Fig. R2-1 (also demonstrated as Supplementary Fig.   15 in the revised version).